The context is: deleting ar pasting a bunch of glyphs in the font overview, and then doing Undo before the write to the backend has completed. This is quite possible with complex designspaces (when a single glyph is stored as many .glif files), since writing glyphs is quite slow (in this backend), although the user generatty doesn't notice as it happens in the background. This caused a variety of problems due to a design flaw and a bug. This PR fixes both.

1. When scheduling writes to the backend, The FontHandler can cause a putGlyph call to be skipped it it is succeeded by a deleteGlyph. This would cause a KeyError if the glyph didn't exist in the first place. Change the behavior of deleteGlyph() so it doesn't raise KeyError when passed an unknown glyph.
2. The external file watch logic would in some cases emit a "reload glyph map" message when the change was in fact caused by the backend itself. This causes unnecessary reloads from the front end with bad timing, and causing the undo stack to be reset unnecessarily.